1. Field of the Invention
The present invention relates to an apparatus for controlling an air-fuel ratio for an internal combustion engine. More particularly, the present invention pertains to an apparatus for controlling an air-fuel ratio for an internal combustion engine such that the air-fuel ratio coincides with a stoichiometric air-fuel ratio, and when the engine is under a predetermined operating condition, the air-fuel ratio is maintained at the leaner side of the stoichiometric air-fuel ratio.
2. Description of the Related Art
To simultaneously reduce the amounts of HC, CO and NO.sub.X in exhaust gas, one type of air-fuel ratio feedback control has heretofore been practiced in which a basic fuel injection quantity is determined on the basis of an engine load (e.g., an intake-air quantity per revolution of an engine or an intake-pipe pressure) and a rotational speed of the engine, and this basic fuel injection quantity is corrected in accordance with the output of an O.sub.2 sensor which detects the residual oxygen concentration in the exhaust gas. In this air-fuel ratio feedback control, a partial-lean control is effected for the purpose of decreasing the rate of fuel consumption and of reducing the amounts of HC and CO in the exhaust gas. More specifically, when the engine operation is under such a condition that the engine load and the rotational speed of the engine are within a predetermined region where the amounts of noxious components in the exhaust gas are relatively small, the feedback control is suspended and switched to a partial-lean control in which the air-fuel ratio is maintained at the leaner side of the stoichiometric level through an open loop control. In the conventional partial-lean control, since misfire occurs very often when the air-fuel ratio is overlean, the leanest value for the air-fuel ratio is set in the vicinity of a misfire region in which misfire may occur and which is determined by both the engine load Q/N and the engine rotational speed N as shown in the specification of Japanese Patent Laid-Open No. 211543/1983. Since the critical air-fuel ratio concerning the misfire region (hereinafter referred to as "critical misfire air-fuel ratio") becomes increasingly leaner than the stoichiometric air-fuel ratio as the engine load and the engine speed increase, the air-fuel ratio can be controlled so as to be increasingly leaner as the engine load and the engine speed increase.
The vibration of a vehicle caused by surges of the engine output overlaps a vibration frequency band to which man is most sensitive when the rotational speed as an output of a transmission is small relative to the rotational speed of the engine (i.e., when the speed reduction ratio is large). Accordingly, when the leanest value in the partial-lean control is set in the vicinity of the misfire region as in the conventional practice, unpleasant surges may be generated due to changes in combustion state particularly when the vehicle is running in the 1st speed gear position, causing driveability to be deteriorated. To overcome this problem, it may be taken into consideration to set the leanest value in the partial-lean control so as to be richer than the critical misfire air-fuel ratio on the basis of the 1st speed gear position. In such arrangement, however, the air-fuel ratio is controlled such as to be richer than the critical misfire air-fuel ratio even when the transmission is set in an intermediate speed gear position or in the top speed gear position where the driveability is not deteriorated by the occurrence of surges because the surge frequency is sufficiently high, resulting disadvantageously in an increase in the rate of fuel consumption.